Particulate matter (particulates) is the generic name used to describe small particles of solid or semi-solid materials, liquid droplets, aerosols and combinations thereof that are present in the ambient air. Total suspended particulates (TSP) refer to those airborne particulates that are less than 100 microns in diameter, which is the approximate thickness of a typical human hair. Particulates less than 10 microns in diameter are designated as PM10. Particulates originate from many sources, including combustion; poor industrial dust containment of such materials as coal dust, fly ash, and carbon black; automotive exhaust, especially diesel; and windblown or fugitive dust from roadways, fields, construction sites, and soil erosion. Photochemical reactions of certain gaseous pollutants in the presence of ultraviolet light also produce airborne particulates in the form of aerosols. These aerosols tend to be less than one micron in diameter. This is smaller in size than either fugitive dust or particulates from industrial sources, which tend to be greater than one micron in diameter.
Particle size distributions can affect visibility, as well as human respiratory functions. Those particles with diameters in the size range of 0.1 to 1.0 microns are the most efficient at scattering visible light which is, generally, light having a wavelength from about 0.4 to 0.7 microns. This scattering is a primary contributor to reduced visibility. When these particles combine with water vapor at high humidity they may, for example, create haze and smog. Larger particles, in the range of 0.5 to 5.0 microns, can be inhaled but are normally deposited in the bronchi before reaching the alveoli or air sacks of the lungs. With the exception of fibrous materials such as asbestos, particles smaller than 0.5 microns, are generally understood to be capable of reaching the avioli.
In addition to reducing visibility and causing respiratory problems, cancer, and heart attacks, airborne particulates can cause corrosion of metals and electrical equipment, as well as soiling of textiles and building materials.
Epidemiological studies in the U.S. and abroad have shown associations between mortality and morbidity and human exposure to ambient particulate matter (Schartz and Dockery, Am. Rev. Resp. Dis. 145:600, 1992; Pope et al., Am. Rev. Resp. Dis. 144:668, 1992). To date, there is limited knowledge about the physical or chemical property of particulate matter that is responsible for these health effects. In addition, there is an increasing interest in developing accurate measurements for particulate matter.
In order to determine air quality, the air may be monitored. Monitoring may be effected in a variety of ways. Typically, samples of air are collected at specific locations for a given period of time, followed by the analysis of the samples using any number of known analytical techniques.
The United States Environmental Protection Agency (EPA) recognizes a need to develop techniques for the continuous measurement of inhalable particulate matter, such as PM10 and PM2.5, which are particulate matter of less than 10 microns and 2.5 microns in diameter, respectively. The majority of the current particulate mass measurement methods use a size selective inlet to remove particles above a certain size, usually 10 microns in diameter (PM10). Most of the available data on PM10 and PM2.5, have been obtained using gravimetric methods. The collected particles are weighed using microbalances under constant specified temperature and relative humidity conditions. However, gravimetric methods are not sensitive enough to measure samples collected in short periods needed to provide a continuous measurement.
The Tapered Element Oscillating Microbalance (TEOM) is a recently developed method that originally appeared to be very promising, as reported in Patashnick and Rupprecht, Continuous PM-10 Measurements Using the Tapered Element Oscillating Microbalance, J. Air Waste Manage. Assoc. 41:1079, (1991). According to this method, the air sample is heated up to 50° C. to remove moisture, and particles are subsequently collected on a TEFLON® filter that oscillates at the top of a metal rod. The amplitude of the oscillation decreases as the mass of the particles collected on the filter increases. Although this method is highly sensitive, measurements are subject to a number of interferences. Significant losses occur for semivolatile organic and inorganic compounds that in some areas can represent relatively large fraction of the total particulate matter. This problem is more pronounced for PM2.5, which includes unstable compounds such as ammonium nitrate and carbonaceous aerosols. For areas such as California and large urban environments, this method may significantly underestimate particle mass concentrations. Also, as the composition of the air sample changes, the partitioning of air pollutants between the gas and particle phase changes. Therefore, absorption and/or desorption processes can take place on the filter, depending upon whether the air sample becomes more or less polluted. Due to the sensitivity of the method, these phenomena can cause either negative or positive artifacts. The gains and losses of mass on the filter are a serious problem, not just of the TEOM, but of any method that collects particles on a filter over a prolonged period of time, for example, over a period of days. Finally, this method presents oscillations or noise in its response. The noise cancels out if a large number of measurements are added to determine a multi-hour concentration estimate; however, over shorter time intervals the measurement errors due to this oscillation can exceed 20–30%.
Short-term measurement of particle size distributions is at least as important as short-term measurement of total particle mass concentrations. In fact, particle size distribution is frequently the most important measurement parameter, since the majority of the physical processes governing the behavior of particles depend on particle size. Particle sizes vary with particle sources, formation mechanisms, and chemical composition. The airborne lifetime of particles is affected by particle sizes. Moreover, the uptake, retention and clearance of particles by the human respiratory system is a function of particle size. Thus, obtaining short-term measurements of particle size distribution, particularly those with diameters smaller than 2.5 microns, could substantially improve exposure assessment and, thus, environmental or regulatory decision-making.
To date, there is no adequate monitoring technique that determines the size distribution of particles based on mass on a continuous basis. Quartz crystal piezobalances are commonly used to determine particulate mass indirectly through particle impaction on an oscillating quartz surface, as reported in Lundgren, D. A., In Fine Particles, edited by B. Y. H. Liu, Academic Press Inc., New, York, (1976); and Chuan, R. L., In Fine Particles, edited by B. Y. H. Liu, Academic Press Inc., New, York, (1976). In these devices, a quartz disk oscillates in an electric circuit at a highly stable resonant frequency which is inversely proportional to the particulate mass impacting and adhering onto the quartz disk.
The piezobalances suffer from the various potential shortcomings. First, the relationship between frequency and mass becomes non-linear for high particulate loadings. Second, since the quartz disk collects particles by impaction, the instrument response depends upon the sharpness of collection efficiency, for example, as affected by the extent of particle bounce and internal particle losses. Finally, carbonaceous aerosol particles, which are composed of long stable chains of very small primary particles, cannot be determined with piezobalances. These chain aggregates contact the sensor at 2 to 3 points, but most of the particulate mass is suspended above the sensor surface where it cannot be measured, for example, as reported by Lundgren, D. A. and Daley, P. S., Am. Ind. Hyg. Assoc. J., 581–588, 1977.
Other direct-reading methods to determine particle concentration and size distribution include optical and electrical counters. Most of the optical systems count light pulses scattered from particles that flow through an intensely illuminated zone. One limitation is the dependence of the instrument's response on the particle refractive index. Measurement error is introduced where particles vary in composition and refractive index. In addition, the art confronts instrument sensitivity problems requiring complex solutions when ambient air contains particles that are larger and smaller than about 0.2 to 0.3 microns, for example, as reported in U.S. Pat. No. 5,835,211 issued to Wells et al.
Another type of particle counter is the aerodynamic particle sizer (APS), for example, Model 3310 sold by TSI Inc. of St. Paul, Minn., which is described by Wilson, J. C. and Liu, B. Y. H., J. Aersol. Sci. 11:139–150 (1980); and Baron, P., Aerosol Sci. and Technol. 5:56–67, (1985). The APS sizes and counts particles by measuring their time-of-flight in an accelerating flow field. Particle measurement is based on particulate inertia, hence the system determines the aerodynamic particle diameter. The main shortcoming of the APS is that it cannot determine size for particles smaller than about 0.7 microns
More generally, electrical counters have been used to determine particle sizes by charging the sampled aerosols and measuring the ability of particles to traverse an electrical field. The most widely used instrument of this type is the Differential Mobility Analyzer (DMA) (Model 3932, TSI Inc., St. Paul, Minn.). This technology is limited to measuring aerosols in the size range 0.01–0.5 microns. Using the DMA in conjunction with an optical counter or the APS would make it possible to determine a broad size range of atmospheric particles. Nevertheless, there are still three other shortcomings. First, both optical and electrical counters determine the number size distribution of particles which they subsequently convert to volume distribution. Since the density of particles varies significantly (in the range of +/−30% of the mean value), and since mass concentration is directly proportional to the density, large uncertainties can result from using these methods to determine particle mass concentrations as a function of size. Second, these techniques require conversion of the size distribution, by number, to a corresponding volume size distribution. The size distribution, by number, of particles is dominated by ultrafine particles (i.e., smaller than 0.1 microns). The coarser the particles, the smaller the concentration becomes. However, when converting a number to volume distribution, a 1.0 micron particle weighs as much as 103 of 0.1 micron particles and 106 of 0.01 micron particles. Counting errors are substantial for large particles, due to their relatively low number concentrations combined with electronic noise in the instrumentation. There are significant uncertainties in volume and, consequently, mass as a function of particle size. Finally, these instruments are very expensive, costing for example up to $100,000 for the combined optical/electrical counter. There are also relatively high maintenance costs, so these devices are generally unsuited for large-scale field studies.
A continuous ambient mass monitor (CAMM) apparatus has been developed at the Harvard School of Public Health, as reported in an Abstract of presentation at conference entitled Measurement of Toxic and Related Air Pollutants, Research Triangle Park, N.C., Cosponsored by the U.S. Environmental Protection Agency and the Air and Waste Management Association, May 7–10, (1995). This apparatus provides for the real time measurement of particulate matter in a gas, and is based on monitoring a pressure drop across a porous membrane filter over a period of time. However, this method is limited to the measurement of the mass of ambient fine particles, generally, less than about 2.5 microns in diameter.
Other particle counters are known, but generally fail to overcome the aforementioned problems of high cost, accuracy and sensitivity. U.S. Pat. No. 5,932,795, issued Aug. 3, 1999, provides a method and apparatus for the continuous monitoring of ambient particle mass in a gas sample using a series of particulate matter collectors where the particle size is from 2.5 to 10 microns. U.S. Pat. No. 5,571,945, issued Nov. 5, 1996, provides a method and apparatus for measuring particulate matter in a gas which employs pressure sensors. U.S. Pat. No. 6,011,479, issued Jan. 4, 2000, provides a personal continuous air monitor capable of sensing radiation. This air monitor employs a filter or detector head with a radiation detector and a series of signal processing units. U.S. Pat. No. 6,187,596, issued Feb. 13, 2001, provides a visual airborne contaminant indicator employing a colored pH indicator, which may be used with an adsorptive filter. U.S. Pat. No. 6,248,153, issued Jun. 19, 2001, provides a diffusional gas transfer system for removing airborne particles.
U.S. Pat. No. 6,431,014 shows a particle measurement device characterized as a cascade impactor. Cascade impactors are generally used for the classification of aerosols according to size and for possible subsequent chemical analysis. Air is drawn through a series of orifices of decreasing size. Air low is usually normal to collecting surfaces on which aerosols are collected by inertial impaction. The particles are separated stepwise by their momentum differences into a number of size ranges, and may be collected simultaneously. One commercially available in-stack cascade impactor is the Graseby Anderson Mark III™ made by Clean Air Analytical Service of Palatine, Ill.
Continuous monitoring of PM concentrations in smoke stacks started during the 1960s in Germany. During the 1970s, studies unsuccessfully attempted to correlate PM concentrations to opacity monitor readings in the United States. More recent studies have evaluated several types of continuous PM monitors including optical, beta gauge, and triboelectric devices. These devices have, with varying degrees of success, continuously monitored total particulate, but have not been used to measure specific size fractions.
It would be a significant contribution to the art to provide an improved, versatile method and apparatus for monitoring the mass of particulate matter in a gas.